The present invention relates to a process for cooling perishable products and, in particular, to a process which does not require moisture resistant product containers.
Several methods are commonly used for cooling perishable products where rapid cooling is required. These include hydrocooling, vacuum cooling, icing and forced air refrigeration.
In the produce field, it is common to pick heads of lettuce and place them in waxed boxes with the box of lettuce then being hosed down with water either before or after the boxes are loaded onto a truck. Although evaporation of water from the lettuce during transportation assists in cooling the lettuce, relatively insignificant amounts of water are absorbed by the waxed boxes and cooling is limited. Transportation of broccoli in waxed boxes filled with ice is also known.
In addition, vacuum cooling approaches have been used for cooling produce. In accordance with this cooling technique, the warm product is loaded into an air-tight chamber or tube which is subsequently evacuated by a mechanical or steam-ejector vacuum pump to establish a partial vacuum therein. As the total gas pressure in the tube is reduced below the saturation pressure of water at the temperature of the warm product (the "flash point"), water on and within the product begins to evaporate rapidly. The thermal energy required to provide the heat of vaporization of this water comes predominately from the sensible heat (e.g. "field heat") of the product. As a result, the product temperature begins to fall as rapid evaporation begins. Because vacuum pumps are generally very inefficient movers of condensable gases, such as water vapor, chilled coils are provided within the tube or chamber to condense and thereby remove the liberated water vapor. These coils are chilled usually by evaporation of liquid ammonia within, the ammonia being supplied by a conventional vapor-compression refrigeration unit.
In the absence of air or any other restriction to water vapor movement from the product to the chilled coil, the temperature of the product will in time equilibrate with that of the coil (the coil temperature in fact being commonly used as a control variable in vacuum cooling operations). Under these circumstances, the rate of thermal equilibration is largely determined by product characteristics. In general, products high in readily evaporated moisture content, with high thermal conductivity and high evaporative surface-to-volume ratio, will cool more rapidly under vacuum than do other types of products. For example, lettuce and other leafy vegetables cool well under vacuum (high moisture content and high surface-to-volume radio), while melons do not (low evaporation rate and low surface-to-volume ratio).
One example of a prior art vacuum cooling system is described in U.S. Pat. No. 4,576,014 to Miller, et al. In these approaches, water has been known to be added to the produce by sprinkling the produce before or while the vacuum is imposed to reduce the amount of moisture removed from the produce during cooling with the water evaporated during cooling being supplied at least in part by the water added to the system instead of entirely by the produce. In these approaches known to the inventor, the vacuum cooled produce sprinkled with water has been packed in waxed boxes which absorb very small amounts of water. This approach requires the use of water resistant containers for the produce.
Therefore, a need exists for a new cooling method for overcoming these and other disadvantages of the prior art.